One type of operational transconductance amplifier (OTA) is the voltage controlled current source (VCCS) whose differential input voltage produces an output current. There is usually an additional input for a current to control the amplifier's transconductance (gm) such that the transconductance of the cell is equal to a fixed current output divided by a fixed voltage input. This type of amplifier can be used in circuits that perform mathematical functions, or “operations” on input signals to obtain specific types of output signals.
A 200-MSample/s Trellis-Coded PRML Read/Write Channel with Analog Adaptive Equalizer and Digital Servo by Alini, et al. (1997), describes, as an example, a fully integrated partial response maximum likelihood (PRML) read/write IC with analog adaptive equalization operating up to 200 MSample/s. The chip implements both matched spectral null (MSN) trellis and standard PR4 Viterbi detectors, in the digital domain as well as digital servo. The device is integrated in a 0.7-/spl mu/m BiCMOS technology, has a die size of 54 mm/sup 2/, and dissipates 2 W with MSN code or 1.5 W with PR4 code at 4.5-V supply and 200 MSample/s. The VCCS OTA described utilizes a transconductance equal to a fixed output ΔI divided by a fixed input ΔV.
Operational Transconductance Amplifier-Based Nonlinear Function Syntheses by Sanchez-Sinencio (1989), describes how the operational transconductance amplifier can be efficiently used for programmable nonlinear continuous-time function synthesis. Two efficient nonlinear function synthesis approaches are presented. The first approach is a rational approximation, and the second is a piecewise-linear approach. Test circuits have been fabricated using a 3-μm p-well CMOS process.
The OTA described by Sanchez-Sinencio may be used for arithmetic functions, such as a multiplier or a divider block. For a multiplier block, a two input four-quadrant multiplier has an output current given by:I1=Km×V1×V2where V1 and V2 are the input voltages, and Km a multiplier constant.The signal level in the multiplier is restricted by a few hundred millivolts for V1 and V2 and Km is a process- and geometry-dependent constant.
For a divider block, Sanchez-Sinencio describes a two-input divider with a voltage output given by:VO=Kr×V1/V2where V1 and V2 are the input voltages and Kr is a constant, also dependent on process and temperature. The resulting output signal is proportional to the ratio of the input signals.
The transconductance of the OTA in Sanchez-Sinencio is fixed and dependent on process parameters, supply variance, and temperature. The output current depends on the square-law principal, making it invalid for state of the art CMOS processes. What is needed is an adaptive transconductance cell that can be used in an operational transconductance amplifier that is simpler, more compact, and immune to these secondary effects.